Embodiments of the invention relate generally to a thermal management system, and more particularly, to a thermal energy storage and transfer assembly for gathering and dispersing radiant thermal energy and kinetic energy of electrons, such as within an electron beam generating device.
Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. Typically, an x-ray beam generating device or x-ray tube comprises opposed electrodes, a cathode and an anode, enclosed within a cylindrical vacuum vessel. A hot cathode filament emits thermal electrons that are accelerated across a typical voltage difference of 20 kV to 200 kV and impact the target zone of the anode at high velocity. The primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location.
However, less than 1% of the primary electron beam energy is converted into x-rays. The balance of the beam energy is contained in back scattered electrons or converted to heat. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses.
Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700° C., while the temperature in the other parts of the anode may range up to about 1800° C.
The excessive temperatures that build up within the x-ray tube can decrease the life of the transmissive window, as well as other x-ray tube components. Due to its close proximity to the focal spot, the x-ray transmissive window is subject to very high heat loads resulting from thermal radiation and back scattered electrons. The high heat loads cause very large and cyclic stresses in the transmissive window and can lead to premature failure of the window and its hermetic seals.
Some methods to address thermal loads in x-ray tubes rely on quickly dissipating thermal energy by using a circulating, coolant fluid within structures contained in the vacuum vessel. Other methods have been proposed to electromagnetically deflect back scattered electrons so that they do not impinge on the x-ray window. These approaches, however, often do not adequately minimize thermal stress on the transmissive window.
Therefore, it would be desirable to design an thermal energy management and transfer assembly that thermally and mechanically isolates the transmissive window in order to minimize thermal stress on the transmissive window.